Known fuel systems for aircraft gas turbine engines include motive flow systems which use motive flow ejector pumps to draw fuel from fuel tanks for delivery to the engines. In aircraft applications where fuel tanks are often located in the wing, however, the fuel drawn out of the wing fuel tanks and directed towards the engines by such motive flow fuel systems is often cold. As a result, ice crystals caused by frozen water droplets produced by condensation created when the fuel is exposed to low temperatures are prone to forming in the cold fuel.
To prevent this from occurring, hot motive fuel systems are often used, wherein the high pressure motive fuel flow used to drive a motive flow ejector pump is bled downstream from an engine fuel/oil heat exchanger. Consequently the bled motive fuel flow is hot when conveyed through conduits of the fuel system. This hot motive fuel flow ejected from the ejector pump will melt any ice crystals formed in the fuel from the fuel tanks before delivery to the engine.
However, such hot motive flow fuel systems are not suitable for use in composite airframes, which are made of resin and carbon fiber for example, because the hot fuel temperature in the fuel system conduits passing through the composite airframe may expose the composite material to temperatures that exceed the recommended temperature limits of the material. Simply insulating the hot fuel system conduits is not practical for a number of reasons, including but not limited to the resulting weight penalty.